Dark field describes an illumination technique used to
enhance the contrast in unstained organisms or any samples. It works by
illuminating the sample with light that will not be collected by the objective
lens and thus will not form part of the image. This produces the appearance of
a dark, almost black, background with bright objects on it.
Principle:
It creates a contrast between the object and the surrounding
field so that the background is dark and the object is bright. The objective
and the ocular lenses used in this microscope are the same as an ordinary
microscope but a special condenser is used that prevents the transmitted light
from directly illuminating the specimen. Only oblique scattered light reaches
the specimen and passes on to the lens causing the bright objects against a dark
background (Fig).
Components:
It has a dark ground condenser that focuses only the oblique
rays of light onto the specimens, a high-intensity light lamp, and a funnel
stop that reduces the apparatus of the objective to less than 1.
Fig: Dark field microscopy
Applications of Dark Field Microscopy:
1. It is used for the diagnosis of Syphilis.
2. Viewing bacteria, algae, and blood cells.
3. Viewing hairline metal fracture.
Advantages of Dark Field Microscopy:
1. It is ideal for viewing unstained, transparent, and
little-absorbed objects.
2. It is ideal to study marine organisms such as diatoms,
algae, plankton, etc.
3. It is used for research on live bacteria, mounted cells,
and tissues.
4. It is used to examine external details like outlines,
edges, grain boundaries, etc.
Disadvantages of Dark Field Microscopy:
1. The images are prone to degradation and distortion.
2. It needs an intense amount of light to work.
3. It is not a reliable tool to obtain accurate measurements
of specimens.
4. If oil or water is used on the condenser then it is
impossible to avoid air bubbles on slides.